Hello fellow duellists!
Side decking is a very important section of Yu-Gi-Oh gameplay and understanding its importance will help you learn to become a better duellist.
Having only 15 cards to choose from to help gain the advantage in your second and possibly third game out of the single match can strengthen your chances of winning the match. Also it can assist you from going from a possible 2-0 defeat into a 2-1 Victory!
Here are some of the key aspects of side decking

What is being played in the current meta-game and what archetypes do you expect to see at this particular event?

The Yu-Gi-Oh TCG is always changing and flowing with new cards, new ideas and every 6 months, a new restriction on what cards you can and cannot play and how many copies of a certain card can be played. Knowing what type of decks are currently seeing play will help you learn about what cards you wish to put into your side deck. The particular event will then justify what cards you may want to side in and how many copies of that particular card.

For example if you are going to a local tournament and you know that a high percentage of the players who regularly attend choose to play “Six Samurai” you may decide to side deck 2-3 copies of “Kinetic Soldier” or “Puppet Plant” or “Gozen match."
All of these cards are aimed towards hurting the “Six Samurai” archetype and will give you a good advantage against this archetype. If you decided to side deck 2 of each of these cards, meaning 6 cards of your side deck is dedicated towards this archetype meaning you have a stronger possibility of defeating them.

However if you are side decking for a larger scale event such as a World Championship Qualifier (WCQ), Battle of the Kingdoms (BOTK) or Nationals, then your side deck will be a lot more difficult to construct. In a larger scale event, you can only go on your gut feeling of what decks you expect to see on that particular event. This means you have to try to make your side deck as universal as possible or have a smaller percentage of your side deck against particular archetypes.

For example you may feel that your deck may struggle against the “Lightsworn” archetype, but the chances of playing against this particular deck are slim. You can chose to side deck 1 or 2 of the following; “Light Imprisoning mirror”, “Leeching the Light”, “Soul Release”. You may even run the risk of side decking 0 copies of these cards and just pray/hope that you don’t face this archetype at the event. However there are universal cards that can help counter that particular archetype.

You may choose to side deck many cards that are “universally” side decked such as “Bottomless Trap Hole”, “Dimensional Prison” or “Smashing Ground”. These cards have no real dedication towards hurting a particular archetype, but are dedicated towards monster removal. Lightsworns not being on the field means that there are less chances of your opponent milling (sending cards from their deck to the graveyard). This will then increase your chances of your opponent not special summoning “Judgement dragon” and leave your opponent with possible dead draws. This will help you in for your chances to victory!

What particular cards that are in my main deck are good and bad against a particular archetype?

The Meta game has a vast amount of archetypes, meaning that there may be cards that are in your main deck that may hinder you or be unsuccessful at combating your opponent. Currently “Maxx C” and “Effect Veiler” are cards that may be in your main deck to combat your opponent from special summoning vast amount of monsters in one turn or reduce the chances of that from happening.

If your opponent is playing an “Anti Meta” or “Stun” deck, they will most likely only be summoning 1 monster per turn and using spell and traps to counter your main deck strategy. This means that your copies of “Maxx C” are bad against this archetype because the probability of your opponent special summoning is very low. Also if you know your opponent is playing cards which banish cards such as “Macro Cosmos” or “Dimensional Fissure”, then your copies of “Effect Vieler” and “Maxx C” cannot be activated.

This means you will side out your copies of “Maxx C” and “Effect Vieler” for cards that you may feel will be better suited against that particular deck.
Also you will need to consider what cards that your opponent will side deck, which may make copies of a particular card in your main deck from being useless, which will be covered in the next segment.

What will my opponent side deck against me?

Knowing what cards your opponent may side deck against you will help you in knowing what cards to side deck to counter them. This means that cards that are in your side deck can have multiple uses against a variety of different archetypes and can serve a different purpose in helping your victory.
For example “Cyber Dragon” could be considered a universal side decked card. It can help you against decks which use Machine types as you can use “Cyber Dragon” to contact fusion into a “Chimeratech Fortress Dragon”, meaning that your opponent’s monsters can be used against them. Since I play the Six Samurai archetype myself, Cyber Dragon has never left my side deck for a good reason. As mentioned earlier “Kinetic Soldier” is a machine type monster, which can be deadly against warrior type monsters such as my Samurais. “Cyber Dragon” can contact fusion with it, meaning it can clear it away for my Samurais to attack other monsters and deal life point damage. Personally I’ve always kept 2 copies in my side deck, so that I can counter “Kinetic Soldier” and any machine decks that I may face.

Another example is that you are playing the “Lightsworn” archetype and you expect your opponent to side deck “Leeching the Light”, so you may decide to side deck “Threatening Roar”. This means that when your opponent plays “Leeching the Light”, they will not be able to attack with all their boosted monsters attack. Also it can aid you against the “Gladiator Beast” archetype since they cannot battle that turn and it can still be used if you want to keep your “Lightsworns” and possible side decked monsters such as “Thunder King Rai-oh” from being destroyed in battle.

Also your opponent may side deck cards such as “Macro Cosmos”, “Banisher of the Radiance” and “Dimensional Fissure”. This means that your copies “Beckoning Light”, “Necro Gardna” and “Monster reincarnation” are less likely to be successful. In this case you would take these cards out of your main deck and replace them with cards that can assist you such as “Mystical Space Typhoon”, “Imperial Iron Wall”, “Dust Tornado” and “Royal Decree”.
All those cards can help you in different methods, each having their own advantages and disadvantages. You may feel that “Royal Decree” may help you since it shuts down your opponent’s traps that they may side in against you such as “Light Imprisoning Mirror”, “Bottomless Trap Hole”, “Dimensional Prison” and “Fiendish Chain”. However “Mystical Space Typhoon” and “Dust Tornado” destroys both spells and traps, meaning that you can destroy both “Macro Cosmos” and “Dimensional Fissure” with this particular card. Also it can be used to destroy face downs which could be any particular card. In the end it’s all down to your own personal judgment of what cards you want to side deck.

What should I always side out?

This is probably one of the most frequent questions I hear from other duellists. Once you have an understanding of what your opponent is going to side against you and what cards may not be good in the upcoming 2nd and 3rd games, then you will have an idea of what to side out.
Siding out can be difficult, but here is a good way of learning of what to side out.
First of all look at your main deck’s strategy of what it is trying to accomplish, its strengths and weaknesses against particular match ups or even particular cards that your opponent may have in his or her main and side deck.
Also consider what cards that you are going to side in to counter your opponent, this may make other cards in your main deck less effective and may need to be side decked.
For example if you are playing the “Lightsworn” archetype and you are deciding to side deck “Shadow Imprisoning Mirror” to counter your opponent’s deck, then there will be cards in your main deck that are “Dark” attribute. “Sangan” and “Necro Gardna” are both dark attribute monsters which effects cannot activate if you have “Shadow Imprisoning Mirror” in play. However if you are only side decking one copy of this card, do you risk keeping them in your main deck?
Personally if I was playing “Lightsworn”, I would side out all the dark monsters which effects cannot be activated in the graveyard or field, as well as “Gorz, the Emissary of Darkness” since he cannot be played if “Shadow Imprisoning Mirror” is face up on the field. The “Dark” attribute monsters would be taken out of the main deck and replaced with cards from the side deck which are more effective and are not affected by the “Shadow Imprisoning Mirror”.
“D.D. Crow” would be a great addition alongside “Shadow Imprisoning Mirror”, even though it is a “Dark” attribute, it’s effect activates in the hand, so that it can banish a monster from your opponent’s graveyard and is unaffected by “Shadow Imprisoning Mirror”.
If you decide to side deck “Royal Decree”, then you may side out the majority of your trap cards, which reduces the chances of you drawing into trap cards whilst your “Royal Decree” is active on the field.
There may be combo cards in your main deck that may not as useful during games 2 and 3. “Pot of Avarice” and “Pot of Duality” are both cards that may be in your main deck that are used to help draw into your combo pieces. However if your opponent side decks “Thunder King Rai-oh” game 2, then your “Pot of Duality” cannot be activated and your combo will slow down. Also if you are going to side deck into your own “Thunder King Rai-oh”, then you may wish to side out this card.

Am I going first or second in this game?

Knowing if you are going to begin the 2nd or 3rd duel may affect what cards you wish to keep in your main deck and what cards you may swap around from your side deck.
Particular cards may be better than others when you are going first or second. Some cards can be reactive, which they are better when you are going second and others may not be as flexible meaning that you want to go first and hope to draw into them to help you.
For example when “Trap Dustshoot” was limited to 1 in the previous forbidden/restricted list it was a card which many players would side out if they had won the 1st duel and possibly put back into their main deck if they had lost the 2nd game and beginning game 3.

“Thunder King Rai-oh” is a prime monster that is used in many players’ main decks or side decks. Usually you wish to play it 1st turn if you draw into him with added protection with your spell and traps. This means that you are less likely to let your opponent have a first turn synchro summon or XYZ summon and at 1900 attack points, he is a solid monster.
However if you are going 2nd, “Thunder King Rai-oh” cannot prevent a special summon, since he is not on the field. So it is a great first turn play for you if you are going first, but not as good if you are going 2nd.

Usually if I am going 2nd in a particular game, I will side deck into cards which are reactive so that it forces my opponent to waste resources.

For example “Ryko, the Lightsworn Hunter” is a reactive card, as you can set the monster and it forces your opponent to usually attack into it and you can destroy their monster. This can be a great card for countering strong first turns and has the flexibility of destroying both monsters and spell/traps. If your opponent has a monster on their field with 1 set spell/trap and they attack into your face down “Ryko, the Lightsworn Hunter”, you can destroy their spell/trap which allows your next summon to be more successful or you may want to destroy their monster as it may hinder your next play.
Other reactive cards which you may wish to consider can be “Gorz, the Emissary of Darkness”, ”Tragoedia” ,”Snow-man Eater”, ” Fossil Dyna Pachycephalo”

Remember that your side deck is all down to your judgment and your ideas of how to counter your opponent. I hope that you enjoyed this article and any feedback/comments and questions will be answered.
Renpuu